1. Field of the Invention
The present invention relates to a mobile communication unit that receives communications services from a cellular communication system if the unit is located within the service area of the cellular communication system and receives communications services from a satellite communication system if the unit is outside the service area. The invention also relates to a mobile communication system utilizing such a mobile communication unit.
2. Description of the Related Art
A dual-mode mobile communication system using dual-mode mobile communication terminals capable of connecting with both a cellular communication system and a satellite communication system has been heretofore available as a mobile communication system for offering wide-coverage mobile communications services to users. Each mobile communication terminal used for such a dual-mode mobile communication system is designed, for example, to detect the reception levels at the systems, to preferentially select a communication system with higher reception power, and to register its location or request connection with the network.
FIG. 11 is a block diagram of the related art mobile communication terminal described in Japanese Unexamined Patent Laid open No. 116949/1997. In FIG. 11, a satellite-based mobile station transmitter 55 and a satellite-based mobile station receiver 56 are connected with a satellite-based mobile station antenna 72 via a transmit-receive switch (DUT) 80. A ground mobile station transmitter 57 and a ground mobile station receiver 58 are connected with a ground mobile station antenna 73 via a transmit-receive switch (DUT) 81. The outputs of a control signal generator circuit 91 and of a voice encoder circuit 92 are connected with the inputs of the transmitters 55, 57, respectively. The outputs of the receivers 56 and 58 are connected with the inputs of a control signal analyzer 93 and of a voice decoder circuit 94, respectively. The input of the voice encoder circuit 92 and the output of the voice decoder circuit 94 are connected with a handset 96. The ground mobile station receiver 58 is connected with a reception level detector circuit 95 whose output is connected with a mobile phone controller 90.
If this mobile communication terminal 30 moves into a location where the reception level of a signal transmitted from a base station of a ground cellular telephone system is in excess of a given level, the location is registered in the base station of the cellular communication system so that the communication mode is switched from the satellite-based communication system to the ground-based communication system. For example, if the reception level detector circuit 95 of the mobile communication terminal 30 detects a reception power exceeding the minimum permissible level of the ground cellular telephone, the mobile communication terminal 30 causes the control signal generator circuit 91 to generate a so-called random access signal (hereinafter referred to as the location registration-requesting signal) for making a request for connection of the ground cellular phone with the network. This location registration-requesting signal is sent to a wireless base station of the ground-based cellular telephone system via the ground mobile station transmitter 57, the transmit-receive switch 81, and the ground mobile station antenna 73.
The mobile communication terminal 30 that is the related art mobile communication unit is constructed as described thus far. The terminal does not make a decision as to whether its present location is within the service area of ground cellular telephone network. The base station of the ground cellular telephone network makes this decision. Even if the reception level detector 95 detects a power exceeding the minimum permissible level for the ground cellular telephone, there is a possibility that the mobile communication terminal 30 is not in the service area of the ground cellular telephone network in practice. Under this condition, if the mode is switched based on the result of the decision made by the reception level detector circuit 95, and if a location registration-requesting signal is sent to the wireless base station of the ground cellular telephone network, then the base station of the ground cellular telephone network rejects it on the grounds that the requested location does not exist within the service area of the ground cellular telephone network. Therefore, there exists a possibility that request for connection of the ground cellular telephone network cannot be made in spite of the cellular processing in the mobile communication terminal 30.
Some mobile communication terminals are designed to operate only one of the satellite-based and ground-based systems because of power saving requirement. Because of the structure, the terminal is urged to receive services of one of these two communication systems at a time. If the above-described situation takes place in the mobile communication system of this construction, it is impossible to make a request for connection with either system. In this way, the continuity of the communications services given to the users using these communication systems has presented problems.